The request cache validator indicated that the response for this request can be served from the cache; however, requests that write data must not use the cache. This exception can occur if you are using a custom cache validator that is incorrectly implemented.

The BeginGetRequestStream method starts an asynchronous request for a stream used to send data for the HttpWebRequest. The asynchronous callback method uses the EndGetRequestStream method to return the actual stream.

The BeginGetRequestStream method requires some synchronous setup tasks to complete (DNS resolution, proxy detection, and TCP socket connection, for example) before this method becomes asynchronous. As a result, this method should never be called on a user interface (UI) thread because it might take considerable time (up to several minutes depending on network settings) to complete the initial synchronous setup tasks before an exception for an error is thrown or the method succeeds.

Your application cannot mix synchronous and asynchronous methods for a particular request. If you call the BeginGetRequestStream method, you must use the BeginGetResponse method to retrieve the response.